A portion of this work is devoted to understanding in detail the significance of covalent bond formation between aminoacyl tRNA synthetases and uridine. The possibility of covalent bond formation between uridine and synthetases is relatively well established, and now the question is whether covalent bond formation with a particular uridine in the tRNA structure (uridine 8) is a crucial part of the mechanism of these enzymes. Affinity labeling projects are also being done with aminoacyl tRNA synthetases, with the goal of isolating active site peptides that can be positioned in the primary structure and, hopefully, in at least one case, within a 3-dimensional structure. In addition, studies, are being done on the way in which charged tRNA regulates the gene expression both in procaryotes and in eukaryotes. In vitro transcription-translation studies are being carried out with defined pieces of cloned DNA, and cloning and sequencing of appropriate eukaryote genes is being done to search for regulatory sequences that would respond to the level of charged tRNA.